$ D = \left[\begin{array}{rrr}0 & 2 & 1 \\ 2 & 1 & 1 \\ 0 & 0 & 1\end{array}\right]$ What is $ D^{-1}$ ?
Solution: $ = \left[\begin{array}{rrr}-\frac{1}{4} & \frac{1}{2} & -\frac{1}{4} \\ \frac{1}{2} & 0 & -\frac{1}{2} \\ 0 & 0 & 1\end{array}\right]$